CALCULATING FIELD WEATHERING RATES USING A MECHANISTIC GEOCHEMICAL MODEL PROFILE

A new model for base cation release due to chemical weathering of soil minerals has been developed based on transition state theory, and inc luded in the integrated soil chemistry model PROFILE. The data require d for model application can be operationally determined on soil sample s, making the model generally applicable and independent of any type o f calibration. The model considers the contribution to the weathering rate from 12 groups of the most common primary and secondary minerals of soils, reacting in separate reactions with H+-ion, H2O, C02 and org anic acids expressed as dissolved organic carbon (DOC). The weathering rate sub-model couples the effects of dissolved Al and base cations o n the reaction mechanisms. The model takes into account changes in soi l temperature, different chemical conditions, the effect of vegetation interactions with the soil and N transformations. The kinetic coeffic ients and reaction orders are based on a complete re-evaluation of wea thering data available in the literature, and additional kinetic data determined by the authors. Data from 23 different independent determin ations of the field weathering rate from 15 sites in Scandinavia, Cent ral Europe and North America were compiled and used to verify the mode l. The model is capable of estimating the release rate of base cations due to chemical weathering from information on soil mineralogy, textu re and geochemical properties of the order of +/-20% of the rate deter mined by independent methods. The results indicate that small amounts of dark minerals like epidote and hornblende, and the plagioclase cont ent, largely determine the field weathering rate.